Methanol purification method and apparatus

ABSTRACT

The invention relates to a method and apparatus to recover and purify methanol from gases produced in the digester during the kraft pulping process. The gas is typically recovered as a foul gas (called stripper off gas or SOG) comprising methanol, water and various other contaminants. The gas is then treated with successive decanting and distillation steps to remove impurities, thereby producing highly purified methanol.

FIELD OF THE INVENTION

This invention relates to a method and apparatus to recover purifiedmethanol stripped from a foul gas stream.

BACKGROUND OF THE INVENTION

Methanol is formed as a by-product of the kraft pulping process, whenthe hydroxyl on reacts with a lignin methoxyl group:

lignin.OCH₃+OH⁻→CH₃OH lignin.O⁻

Depending on the mill configuration, up to 90% of the methanol generatedin the digester can be captured in the foul condensate from the digesterand evaporator areas. The foul condensate is typically treated in asteam stripping system, where up to 95% of the methanol can be removedfrom the foul condensate and captured in the overhead vapours from thestripping process. The concentrated gas stream is often referred to asstripper off gas (SOG).

The SOG is then usually disposed of through thermal oxidation in a limekiln, power boiler, recovery boiler, or dedicated incinerator. The SOGtypically consists of about 40 to 70 wt % methanol, 5 to 10 wt %non-condensable materials, including sulphur compounds, and the balancewater vapour.

Waste SOG can be burned as a replacement for fossil fuels. However, thevalue of SOG as a fuel depends on the amount of water vapour that itcontains. Natural gas provides 50.5 MJ/kg (37.2 MJ/m³) heat ofcombustion, pure methanol provides 22.7 MJ/kg, and SOG containing 70 wt% methanol provides the equivalent of about 21.9 MJ/kg. The SOG providesless heat because the entrained water vapour must first be heated up tocombustion temperature.

Chlorine dioxide is used in the pulp bleaching process; grade AAmethanol (99.85 wt %) is used to manufacture ClO₂. In a well-run mill, amethanol purification system would preferably be able to producesufficient amounts of purified methanol for the demands of the ClO₂process, as well as some purified methanol for external sale. If asubstantial portion of the methanol in the SOG can be recovered andpurified to an industrial grade AA product, the methanol produced in atypical kraft pulping process could be worth as much as four and a halftimes more as a commodity than as a fuel.

There are numerous methanol purification systems in operation. Most suchsystems use some form of distillation to separate methanol from othercompounds. See for example, U.S. Pat. No. 5,718,810 to Robbins and U.S.Pat. No. 6,217,711 to Ryham et al. Canadian Patent No. 1,0888,957 toSuokas et al., uses a combination of distillation steps and acid oralkaline oxidating treatments to separate the various fractions.Distillation separates the components of a solution by partialvapourization of the mixture and separate recovery of vapour andresidual liquid. The more volatile constituents of the original mixtureare obtained in increased concentration in the vapour, while lessvolatile components remain in greater concentration in the liquidresidue. Distillation columns may be designed using trays, structuredpacking, or random dumped packing. Due to restricted access, for smallcolumns below about 750 mm diameter, random dumped packing is preferred.

However, methanol recovered from a kraft pulping process has severalunique characteristics that inhibit separation by distillation.

Typically, significant quantities of dimethyl disulphide are present inthe crude methanol produced during the kraft pulping process. Thepresence of an azeotrope between methanol and dimethyl disulphiderequires that the methanol content in the SOG be no higher thanapproximately 40 wt % to ensure separation. Control of the foulcondensate steam stripping system, in terms of both the quantity andquality of SOG produced, can reduce the impact of azeotropes of dimethyldisulphide. Many existing stripping systems include a reflux condenserintegrated with the multiple effect evaporators; see for example U.S.Pat. No. 4,137,134 to Suominen et al., U.S. Pat. No. 3,807,479 toBrannland et al., and U.S. Pat. No. 5,830,314 to Mattsson.Unfortunately, in this arrangement, control of the stripping system maybe compromised because any fluctuations in evaporator operation willripple through the stripping system, unpredictably affecting SOGquantity and quality.

Further, contaminants including ionizable sulphur compounds such ashydrogen sulphide and methyl mercaptan are produced during the pulpingprocess. These compounds can dissociate under certain conditions, makingthem all but impossible to remove from SOG by simple distillation. Ascan be seen in FIG. 1, hydrogen sulphide (H₂S) begins to dissociate at apH above about 6, while methyl mercaptan (MM) begins to dissociate at apH above about 9. In their dissociated form, these compounds do notexert a vapour pressure and therefore can not be removed bydistillation. Controlling the pH of the liquid phase in the distillationcolumn is therefore an effective way to remove these compounds in adistillation process.

As condensed SOG typically has a pH of about 9 to 10, an acid, such assulphuric acid, may be metered to the appropriate distillation column tolower the pH in the system. However, the acid cannot simply be added tothe liquid feed to the column as it will react with any ammonia presentin the system, producing ammonium sulphate. This is known as fouling thecolumn and is to be avoided. U.S. Pat. No. 5,989,394 to Johansson et al.describes a process in which an acidifier is introduced to a strippingcolumn above the admission point of the liquid being purified, oralternatively is added to the liquid feed directly. However, Johanssonis concerned with producing a relatively purified condensate stream,rather than removal and high level purification of methanol from theliquid feed stream and does not seem to be concerned with fouling thecolumn.

It is therefore an object of the invention to provide a method andapparatus to recover and purify methanol stripped from a foul gas streamthat overcomes the foregoing deficiencies.

In particular, it is an object of the invention to provide a method andapparatus to recover and purify methanol to a high degree, allowingmethanol to be used within a kraft pulping process and to allow excessmethanol to be sold, rather than destroyed.

These and other objects of the invention will be appreciated byreference to the summary of the invention and to the detaileddescription of the preferred embodiment that follow.

SUMMARY OF THE INVENTION

The invention relates to a method and apparatus to recover and purifymethanol from gases produced in the digester during the kraft pulpingprocess. The gas is typically recovered as a foul gas (called stripperoff gas or SOG) comprising methanol, water and various othercontaminants.

Stripper off gas is stripped from the digester and evaporator areas ofthe pulping process; the SOG then passes, at a controllable flow rate toa dedicated condensing means, where volatile components are boiled offand vented to an incineration system, while the condensate drains to atopping red oils removal means, such as a decanter. Heavy contaminantsthat are immiscible in the solution are decanted and recoveredseparately. The underflow is moved to a first distillation means, suchas a topping column and heated. Acid is added to the mid-point of thetopping column to lower the pH of the solution without allowing the acidto react with ammonia in the feed. Volatile components are returned tothe condensing means, while the underflow moves to a surge tank, whichmay be used to stabilize the flow and concentration of the feed to therectification section.

The rectification section may comprise one or two columns. The feed isintroduced near the top of the bottoms section of the column, and movesdown through the packing in the column, countercurrent to the strippingsteam flow. Vaporized methanol moves up through the top section of thecolumn, and any impurities are removed as the overhead vapor flow. Waterand other less volatile components form the underflow, while fusel oilsare drawn off in a side stream. Purified methanol is drawn off andpassed to a methanol cooler for condensation and storage. The methanolis at least 99.85 wt % pure.

Alternatively, the bottoms section and the top section may each be aseparate column. The feed is introduced near the top of the bottomscolumn, and moves down through the packing in the column, countercurrentto the stripping steam flow. Vaporized methanol is removed as theoverhead vapor flow. Water and other less volatile components form theunderflow, while fusel oils are drawn off in a side stream. The methanolvapor is passed to the rectification top column, where it is distilledagain. Condensate from the rectification top column is returned to therectification bottoms column, while vapors are collected and condensedbefore being vented to the incineration system. Purified methanol isdrawn off and passed to a methanol cooler for condensation and storage.The methanol is at least 99.85 wt % pure.

In one aspect, the invention comprises a method to recover and purifymethanol from a stripped off gas stream, comprising the steps of:obtaining, at a controlled rate, a foul gas feed comprising no more thanapproximately 40 wt % methanol; condensing the foul gas feed; removingimmiscible contaminants from the condensed foul gas feed; heating thecondensed foul gas feed in the presence of an acid to evaporate volatilecomponents, leaving a contaminated methanol feed, the acid beingsupplied at an entry point below an input point of the condensed foulgas feed; refining the contaminated methanol feed by heating toevaporate methanol from the contaminated methanol feed; and furtherrefining the evaporated methanol by heating to evaporate remainingvolatile components and to produce purified methanol and impurecondensate. The purified methanol may be cooled and collected forstorage. The condensate may be recycled to the step of refining thecontaminated methanol feed.

In a further aspect, excess foul gas may be diverted to a disposalsystem prior to the condensing step.

In yet a further aspect, fusel oils may be stripped from thecontaminated methanol feed during the refining step.

In further aspect, the invention may comprise the further step ofstoring the contaminated methanol feed prior to refining thecontaminated methanol feed.

In a further aspect, the immiscible contaminants may be removed bydecanting the immiscible contaminants.

In another aspect, the invention comprises an apparatus to recover andpurify methanol from a stripped off gas stream, comprising: condensingmeans to receive and condense a controlled amount of stripped off gascomprising no more than approximately 40 wt % methanol; decanting meansto remove immiscible contaminants from the condensed gas; firstdistillation means comprising upper and lower sections, to receive thecondensed gas in the upper section, and to heat the condensed gas in thepresence of acid received in the lower section, to evaporate volatilecomponents, leaving contaminated methanol; a first refining section toevaporate methanol from the contaminated methanol; and a second refiningsection to evaporate and condense impurities from the evaporatedmethanol, producing purified methanol. Means may also be provided tocapture and condense the purified methanol for storage

In a further aspect, the apparatus of the invention may comprise storagemeans to store the contaminated methanol prior to entering the firstrefining section.

In a further aspect, the apparatus of the invention may comprise meansto remove fusel oils from the contaminated methanol.

In another aspect, the first distillation means of the apparatus of theinvention may comprise a topping column. The topping column may furthercomprise a reboiler to recycle part of the contaminated methanol.

In yet another aspect, the first and second refining sections of theapparatus of the invention may comprise a second distillation means. Thesecond distillation means may comprise a rectification column, or firstand second rectification columns.

In a further aspect, the apparatus of the invention may comprise meansto divert excess gas to a disposal system prior to entering thecondensing means.

The foregoing was intended as a broad summary only and of only some ofthe aspects of the invention. It was not intended to define the limitsor requirements of the invention. Other aspects of the invention will beappreciated by reference to the detailed description of the preferredembodiment and to the claims.

The inventors thank Alberta-Pacific Forest Industries Inc. for itscontinued interest in this work and for its assistance in testing thesystem.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiment of the invention will be described by referenceto the drawings in which:

FIG. 1 is a graph showing the dissociation fractions for hydrogensulphide and methyl mercaptan at various pH levels;

FIG. 2 is a schematic of the topping section of the invention;

FIG. 3 is a schematic of the rectification section of the invention; and

FIG. 4 is a schematic of an alternative layout of the rectificationsection of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Stripper off gas (SOG), typically containing about 40 to 70 wt %methanol, is produced in an existing foul condensate steam strippingcolumn. The SOG is directed to a methanol purification system 10, beingdiverted from a kiln, boiler, incinerator or other incineration system12, as shown in FIG. 2.

Vapour 14 from the existing stripping column is introduced to adedicated reflux condenser 16; this vessel may be of any suitable type,such as a falling film type shell and tube evaporator effect. The heatfrom the stripping system may be utilized in the evaporator system, butuse of a dedicated vessel allows sufficient control over the system toensure stable qualities and quantities of SOG are produced under allevaporator operating conditions. Pressure is maintained by throttlingthe flash vapour from the system.

SOG is introduced to the methanol purification system 10 at a controlledflow rate, with any excess gas being diverted to the incineration system12. This helps to maintain the methanol entering the purification system10 at an optimal content of approximately 40 wt % or less.

The topping column system 18 strips out low boilers and non-condensablesfrom the SOG, including malodorous sulphur compounds, ammonia, and someethers, ketones and aldehydes. When SOG is introduced to the toppingreflux condenser 20, the low boilers and non-condensables are vented 22back to the incineration system 12 while the condensate is drained 24 tothe topping red oils decanter 26.

Topping red oils pump 28 moves the decanted red oils to a turpentinerecovery system (not shown), if available. The underflow 32 from thedecanter 26 is moved to the topping column 34 by any suitable means,such as topping reflux pump 36. A topping reboiler 38 may be used toprovide heat to the topping column 34, evaporating the volatilecontaminants in a stream 42, which can be returned to the topping refluxcondenser 20 or otherwise disposed of.

Sulphuric acid may be added to topping column 34 by any suitable means,such as feed pump 44. Preferably the acid is added about the mid-pointof the column, or at any rate at an entry point 46 below the input point48 of the condensed underflow feed from the topping reflux pump 36. Theseparation between the feed input point 48 and the acid entry point 46allows any highly volatile ammonia present in the underflow feed to bestripped out in the upper section of the topping column 34 before it hasa chance to react with the acid, thereby avoiding the formation ofammonium sulphate precipitates. The acid reduces the pH in the lowersection of the topping column 34, releasing dissociated hydrogensulphide and methyl mercaptan, which will rise to the upper section oftopping column 34, where it can be removed as part of volatilecontaminant stream 42.

The underflow 50 from the topping column 34 flows to the surge tank 52,with some being recycled to topping reboiler 38. As the flow andconcentration of SOG can vary significantly depending on the operationof the existing stripping system, the surge tank can smooth out the flowand concentration of the feed to the methanol rectification columnsystem 54.

The feed enters rectification system 54 from surge tank 52, such as byrectification feed pump 56. The rectification column system 54 comprisestwo sections, namely a bottoms stripping section 97 and a toprectification section 99, as shown in FIG. 3. The feed is introduced tothe stripping section 97 of column 55 and flows down through thepacking, countercurrent to the stripping steam 57, which may be suppliedby a rectification reboiler 59. The volatile component, includingmethanol, moves upward to the top rectification section 99, while theless volatile component, which is mainly water along with other highboilers, is removed as the underflow 63.

The feed may also comprise intermediate boilers, such as some higheralcohols (primarily ethanol), higher ketones, etc. These components,often referred to as fusel oils, are drawn off from the bottoms column55, preferably at a point 65 located below the feed introduction point67. The fusel oils can be recovered separately, or may be combined withthe underflow 63 from the column 55, passing to effluent treatmentthrough rectification bottoms pump 69.

The overhead vapour flow 61, comprising methanol and other volatiles,from upper rectification section 99 is condensed in a rectificationreflux condenser 71, located above column 55. Any low boilers andnon-condensables 73 may be vented to the incineration system 12.

The remaining product, which is approximately 99.85 wt % methanol, isdrawn off in a stream 75, preferably located slightly below the top ofthe packing in top rectification section 99, and moved to a methanolcooler 77 by suitable means such as by methanol pump 79, where it can bemoved to storage. The methanol product is preferably drawn off insufficient quantities to maintain the methanol profile in the column.

Alternatively, the two sections of rectification column system 54 may besupplied in two separate columns, the rectification bottoms column 60and the rectification top column 62, as shown in FIG. 4. The feed isintroduced 64 to the stripping section of the bottoms column 60 andflows down through the packing, countercurrent to the stripping steam66, which may be supplied by a rectification reboiler 68. The volatilecomponent, including methanol, is removed into the overhead vapour flow70, while the less volatile component, which is mainly water along withother high boilers, is removed as the underflow 72.

In this embodiment, the fusel oils are drawn off from the bottoms column60, preferably at a point 74 located below the feed introduction point64. Again, the fusel oils can be recovered separately, or may becombined with the underflow 72 from the column 60, passing to effluenttreatment through rectification bottoms pump 76.

The overhead vapour flow 70 from rectification bottoms column 60 isdirected to the lower section of the rectification top column 62. Anycondensate 80 collected in the bottom of the top column 62 may bereturned by an intermediate rectification pump 82 to introduction point84 of the bottoms column 60. Vapour 86 from the top column 62 iscondensed in a rectification reflux condenser 88, located above topcolumn 62. Any low boilers and non-condensables 78 may be vented to theincineration system 12.

The remaining product, which is approximately 99.85 wt % methanol, isdrawn off in a stream 90, preferably located slightly below the top ofthe packing in top column 62. Again, the methanol product is preferablydrawn off in sufficient quantities to maintain the methanol profile inthe column and moved to the methanol cooler 94 by suitable means such asby methanol pump 92, where it can be moved to storage.

It will be appreciated by those skilled in the art that other variationsto the preferred embodiment described herein may be practised withoutdeparting from the scope of the invention, such scope being properlydefined by the following claims.

1. A method to recover and purify methanol from a stripped off gasstream, comprising the steps of: obtaining, at a controlled rate, a foulgas feed comprising no more than approximately 40 wt % methanol:condensing said foul gas feed; removing immiscible contaminants fromsaid condensed foul gas feed; heating said condensed foul gas feed inthe presence of an acid to evaporate volatile components, leaving acontaminated methanol feed, said acid being supplied at an entry pointbelow an input point of said condensed foul gas feed; refining saidcontaminated methanol feed by heating to evaporate methanol from saidcontaminated methanol feed; and further refining said evaporatedmethanol by heating to evaporate remaining volatile components and toproduce purified methanol and impure condensate.
 2. The method of claim1 further comprising the step of diverting excess foul gas to a disposalsystem prior to said condensing step.
 3. The method of claim 1 furthercomprising the steps of cooling and collecting said evaporated methanol.4. The method of claim 1 further comprising the step of stripping fuseloils from said contaminated methanol feed during said refining step. 5.The method of claim 1 further comprising the step of storing saidcontaminated methanol feed prior to said step of refining saidcontaminated methanol feed.
 6. The method of claim 1 further comprisingthe step of recycling said condensate to said step of refining saidcontaminated methanol feed.
 7. The method of claim 1 wherein said stepof removing immiscible contaminants comprises decanting said immisciblecontaminants.
 8. An apparatus to recover and purify methanol from astripped off gas stream, comprising: condensing means to receive andcondense a controlled amount of stripped off gas comprising no more thanapproximately 40 wt % methanol; decanting means to remove immisciblecontaminants from said condensed gas; first distillation meanscomprising upper and lower sections, to receive said condensed gas insaid upper section, and to heat said condensed gas in the presence ofacid received in said lower section, to evaporate volatile components,leaving contaminated methanol; a first refining section to evaporatemethanol from said contaminated methanol; and a second refining sectionto evaporate and condense impurities from said evaporated methanol,producing purified methanol.
 9. The apparatus of claim 8, furthercomprising storage means to store said contaminated methanol prior toentering said first refining section.
 10. The apparatus of claim 8,further comprising means to capture and condense said purified methanolfor storage.
 11. The apparatus of claim 8, further comprising means toremove fusel oils from said contaminated methanol.
 12. The apparatus ofclaim 8, wherein said first distillation means comprises a toppingcolumn.
 13. The apparatus of claim 12, wherein said topping columnfurther comprises a reboiler to recycle part of said contaminatedmethanol.
 14. The apparatus of claim 8, wherein said first and secondrefining sections comprise a second distillation means.
 15. Theapparatus of claim 14 wherein said second distillation means comprises arectification column.
 16. The apparatus of claim 14, wherein said seconddistillation means comprises first and second rectification columns. 17.The apparatus of claim 8, further comprising means to divert excess gasto a disposal system prior to entering said condensing means.